Diagnosing Iron Deficiency Anemia

Introduction to Diagnosis

The World Health Organization criteria for anemia are hemoglobin <13.0 g/dL in men, <12.0 g/dL in non-pregnant women, <11.0 g/dL in pregnant women, and <11.5 g/dL in children 5-11 years 1.

Hemoglobin is preferred over hematocrit for diagnosing anemia because it has better reproducibility across laboratories and is not affected by storage time or patient variables like serum glucose 3.

The essential initial tests for diagnosing iron deficiency anemia include complete blood count with red cell indices, serum ferritin, transferrin saturation, and inflammatory markers, according to the Journal of Crohn's and Colitis 1, 2, 4, 5, 6, 7.
The American Gastroenterological Association recommends a complete blood count with red cell indices (MCV, MCH, RDW) for the initial evaluation of anemia 5.

Mean corpuscular volume (MCV) provides the first clue for differentiating between iron deficiency anemia and other types of anemia, with microcytosis suggesting iron deficiency, thalassemia, or anemia of chronic disease 1, 4, 6.
A high red cell distribution width (RDW) indicates iron deficiency even when MCV appears normal due to coexisting microcytosis and macrocytosis 4, 6.

In the absence of inflammation, serum ferritin <30 μg/L is diagnostic of iron deficiency, while transferrin saturation <16% supports iron deficiency 1, 4, 5, 7, 8.
With inflammation, higher ferritin thresholds are required, and serum ferritin <100 μg/L may still indicate iron deficiency 1, 4, 5, 6, 7.

If the cause of anemia is not evident after initial testing, further evaluation should include vitamin B12 and folic acid levels, haptoglobin, lactate dehydrogenase, differential white blood cell count, and percentage of hypochromic red cells or reticulocyte hemoglobin content 1, 2, 3, 5.
Soluble transferrin receptor (sTfR) can help differentiate between iron deficiency and anemia of chronic disease, with elevated levels indicating iron deficiency 7, 9.

In patients with chronic inflammatory conditions, such as inflammatory bowel disease, chronic kidney disease, heart failure, or cancer, higher ferritin thresholds (<100 μg/L) should be used for diagnosis, and functional iron deficiency should be considered 1, 4, 5, 7.
Pregnant women should be screened for iron deficiency, as up to 84% may develop it by the third trimester 10.